There is (1)the answer, and (2)why you should just trust us on it even if you don’t understand.
You should just trust us on it, the same way you will acknowledge that women are incredibly “petty” in many ways–as men perceive it. But that those behaviors are actually not petty for women. Even though men will never ever understand.
The answer is that the way men are made, is that looks are very important when beginning a relationship, and that in particular, being thin is very attractive. And that is the truth–don’t let any woman convince you otherwise.
The reason G-d did that, is because that way men are attracted to health. In our society, being too fat is a bigger health problem than being too skinny, so thin is perceived as healthy.
And if you think about it, that is why G-d made you guys so absurdly petty about your looks.
I once posted an article about this, written for women. It is in one of these threads, I can try to look for it later.